The present invention relates to oxygen-absorbing materials excellent in their oxygen-absorbing property and suitable for use in the form of powder, particles or film or for use as one component of a molded product such as a container. The present invention further relates to containers and molded products excellent in their gas barrier property that use the oxygen-absorbing materials therein. More specifically, the present invention relates to containers and molded products excellent in their gas barrier property and accordingly suitable for preserving food items/beverages or pharmaceutical drugs that are damaged by oxygen in terms of their taste, oxygen-sensitive substances and the like.
Thermoplastic polyester resin which includes polyethylene terephthalate as a major component has been used widely for packaging materials by being processed into various containers, films, sheets and the like, because of its superior dynamic properties, gas barrier property, chemical resistance, flavor-retaining property, hygienic properties and the like. However, even the thermoplastic polyester resin including the polyethylene terephthalate as a major component does not have absolutely perfect properties. Especially, the thermoplastic polyester resin is not suitable for containers of food items, beverages, cosmetics, pharmaceutical drugs and the like that are contents requiring a gas blocking property, since the thermoplastic polyester resin is particularly insufficient in a gas barrier property against oxygen. Accordingly, improvements have been made by blending the polyester resin including the polyethylene terephthalate as a major component with any gas barrier material having an excellent gas barrier property or layering them on each other to produce a laminate. However, the improvements are not appropriate for food items and beverages sensitive to oxygen. Moreover, National Patent Publication No. 2-500846 and Japanese Laying-Open No. 3-762 disclose techniques according to which molded products such as container, package and lid are produced by blending the polyester resin with MXD6 (condensation polymer of metaxylylene diamine and adipic acid) which is oxidizable polyamide resin and a Co compound which is oxidation catalyst, or by layering them on each other to produce a laminate. Although the techniques provide improvements to some degree, they are still inappropriate for food items, beverages, pharmaceutical drugs and cosmetics that are sensitive to oxygen.
One object of the present invention is to provide an oxygen-absorbing material superior in its ability to preserve food items, beer, soft drinks, cosmetics, drugs or the like that are sensitive to oxygen for a long period of time without damage, which has not been achieved by any prior art, and to provide a molded product such as packaging container with the gas barrier property using the oxygen-absorbing material therein.
According to the present invention achieving the object above, a packaging container or molded product having the gas barrier property can contain a composition structured by blending a non-oxidizable thermoplastic resin with at least one type of oxidizable polymer having an oxygen-capturing function and a metal catalyst catalyzing oxidation of the oxidizable polymer or layering them on each other to form a laminate.
Here, the oxidizable resin refers to a resin oxidized through an oxidization reaction in the presence of oxygen. Specifically, the oxidizable resin has a structural unit including a methylene group bonded to an aromatic ring, the methylene group being further bonded to an element other than hydrogen and carbon, for example. Examples of the element other than hydrogen and carbon are N, O, S and the like preferably having a xylylene diamine structure.
It is particularly important that the content of the blended or layered oxidizable resin to form the container is at least 1 wt % with respect to structural components and that a polyamide resin is used containing a basic group such as amino group of 20 mmol/kg or less. Examples of the metal catalyst are compounds of Fe, Mn and Co that are transition metals of the first period. These metal catalysts can be included in advance in the oxidizable resin before being molded. The present invention is described below in more detail together with functions thereof
The inventors of the present invention have found that an amino group contained in the oxidizable resin has an influence on the oxygen-capturing function of the oxidizable resin which is subjected to an oxidation reaction by the metal catalyst. The inventors have accordingly found a resin with the gas barrier property excellent in the oxygen-capturing ability, by controlling the amount of such functional groups within an appropriate range. According to findings of the inventors, the amount of the basic group such as amino group included in the oxidizable resin is 20 mmol/kg or less, preferably 15 mmol/kg, and more preferably 10 mmol/kg or less, so that an excellent oxygen-capturing ability is achieved with the help of the metal catalyst such as Co.
The oxidizable resin used for the present invention is preferably a polyamide and more preferably a polyamide resin containing a metaxylylene group. A particularly preferable polymer contains, in a molecular chain, at least 70 mol % of a structural unit formed of metaxylylene diamine or mixed xylylene diamine including metaxylylene diamine and paraxylylene diamine of 80% or less with respect to the entire amount and xcex1, xcfx89 aliphatic dicarboxylic acid with the carbon number of 6-10. Examples of the polymer are homopolymers such as polymetaxylylene adipamide, polymetaxylylene sebacamide and polymetaxylylene superamide, copolymer of metaxylylene diamine/adipic acid/isophthalic acid, copolymer of metaxylylene/paraxylylene adipamide, copolymer of metaxylylene/paraxylylene piperamide, copolymer of metaxylylene/paraxylylene azelamide, and the like.
These polyamide resins containing the metaxylylene group are brittle in an amorphous state. Therefore, such resins are required to have a relative viscosity of preferably at least 1.5 and more preferably at least 2.0.
It is necessary that the amount of the amino group (AG) contained in the oxidizable polyamide is 20 mmol/kg or less. The amount of amino group can be adjusted by adding an excessive amount of dicarboxylic acid component to the diamine component in the process of polymerization, or by adding any sealing agent reacting with the amino group such as acid anhydride, monocarboxylic acid and the like when the polymerization process is completed, for example.
The amount of carboxyl end group contained in the polyamide resin is preferably at least 10 mmol/kg and more preferably at least 20 mmol/kg.
Preferably, the molar ratio of the carboxyl end group (CEG) and amino group (AG) contained in the polyamide resin, i.e., CEG/AG is at least 2, or at least 4, so that the oxygen capturing ability obtained by the metal catalyst such as Co can be enhanced.
According to the present invention, in addition to the oxidizable resin, a non-oxidizable thermoplastic resin is preferably used for forming a molded product such as container by blending or layering of them. Examples of the non-oxidizable thermoplastic resin are polyester resin, polyamide resin such as nylon 6 and nylon 66, polymer with a high nitrile content, copolymer of ethylene-vinyl alcohol, polycarbonate, polystyrene resin, and the like. The non-oxidizable thermoplastic polyester resin refers to a polyester usually containing at least 80 mol %, preferably at least 90 mol % of terephthalic acid in an acid component and containing at least 80 mol %, preferably at least 90 mol % of ethylene glycol in a glycol component. The remaining portion of the acid component includes for example isophthalic acid, diphenyl ether 4,4xe2x80x2-dicarboxylic acid, naphthalene 1,4- or 2,6-dicarboxylic acid, adipic acid, sebacic acid, decane 1,10-dicarboxylic acid, or hexahydroterephthalic acid. The remaining portion of the glycol component includes for example propylene glycol, 1,4-butanediol, neopentyl glycol, diethylene glycol, cyclohexane dimethanol, or polyethylene naphthalate, or copolymer thereof. The polyester resin further contains a hydroxy acid such as p-oxy benzoic acid. Alternatively, at least two types of polyesters may be blended so that the ethylene terephthalate is contained in the range as described above. The thermoplastic polyester resin according to the present invention has an intrinsic viscosity of preferably at least 0.55, and more preferably in the range from 0.65 to 1.4. The intrinsic viscosity less than 0.55 is not enough for a resultant molded product such as container to have a sufficient mechanical strength.
Specific examples of the non-oxidizable polyamide resin used for the present invention are thermoplastic polyamide resin such as polycapric amid (nylon 6), polyundecanamide (nylon 11), polylaurinlactam (nylon 12), polyhexamethylene adipamide (nylon 6, 6), polyhexamethylene sebacamide (nylon 6, 10), caprolactam/laurinlactam copolymer, and caprolactam/hexamethylene diammonium adipate copolymer, as well as blends of these homopolymers or copolymers, for example. These non-oxidizable polyamide resins have a relative viscosity of preferably at least 1.5, and more preferably at least 2.0. According to the present invention, the polymer with a high nitrile content employed as the non-oxidizable thermoplastic resin is a thermoplastic copolymer which contains 40 to 97 mol %, with respect to the entire polymer, of ethylene-based unsaturation monomer containing a nitrile group such as acrylonitrile, methacryl nitrile and mixture thereof, and contains, as a remaining copolymer component, 3 to 60 mol % of at least one or a combination of at least two types of monomer such as styrene, vinyltoluene, butadiene, isoprene, methyl methacrylate methyl acrylate, and methylvinylether. An example of the ethylene-vinyl alcohol copolymer used as the non-oxidizable thermoplastic resin according to the present invention is saponified copolymer of ethylene and vinylester such as vinyl formate, vinyl acetate and vinyl propionate, for example.
According to the present invention, if the molded product such as packaging container has a multilayer structure formed of laminate, a blend of the non-oxidizable thermoplastic resin and oxidizable resin may constitute one layer. Further, any additive such as coloring agent, ultraviolet absorber, antistatic agent, lubricant and the like may be contained in any appropriate layer with an appropriate content.
According to the present invention, preferable examples of the metal catalyst are compounds of transition metals of the first period (Fe, Mn, Co, Cu) and rhodium. Specifically, organic salt, chloride, phosphate, phosphite, hypophosphite, diphosphate, methaphosphate, sulfate, alkyl phosphate or phthalocyanine complex only, or mixture thereof is used. Examples of the organic salt are the salts of acetic acid, propionic acid, octanoic acid, lauric acid, and stearic acid that are salts of aliphatic alkyl acid of C2-C22, and the salts of malonic acid, succinic acid, adipic acid, sebacic acid, and hexahydro phthalic acid that are dibasic acid salts, the salts of butane tetra carboxylic acid, benzoic acid, toluic acid, o-phthalic acid, isophthalic acid, terephthalic acid and trimesic acid that are aromatic carboxylic acid salts, and the salts of polyacrylic acid and ethylene/acrylic acid copolymer that are macromolecular carboxylic acid salts, and they are used solely or as a mixture thereof.
A packaging container of the structure according to the present invention was produced and effects thereof were examined. As a result, it has been found that the container using the thermoplastic resin therein, which has not been achieved by prior arts, is appropriate as a molded product such as packaging containers of oxygen-sensitive food items and beverages.
The molded product here refers to products in the forms of cloth, nonwoven fabric, film, and sheet, and further to containers such as bag and hollow molded product, tray, and lid constituting a part of a container or bag.
The inventors of the present invention have closely studied the oxygen-absorbing ability to find that, in a polymetaxylylene adipate (MXD6), an oxidation reaction of the MXD6 of capturing oxygen molecules predominantly occurs in an amorphous portion thereof and the reaction proceeds considerably slowly in a crystalline portion thereof. One reason for this is considered that oxygen absorption occurs in the crystalline portion only at the surface and accordingly the oxygen is less likely to enter the inside, or, if the MXD6 is in a readily-crystallized state, the crystallization speedily occurs in a portion absorbing oxygen and thus the crystallized portion serves as a gas barrier layer preventing oxygen from entering, and consequently, oxygen is not absorbed efficiently.
According to this finding, a copolymer component which lessens the crystallinity of oxidizable resin represented by MXD6 is incorporated in the resin or the resin is blended with another resin in order to reduce the crystallinity of the oxidizable resin. In this way, a polymer material superior to conventional oxygen absorbers in the oxygen-absorbing ability can be obtained.
Further, the present invention is an oxygen-absorbing material formed of a resin composition including a resin component and a metal catalyst. The resin component includes an oxidizable resin which is at least one of a resin having a melting point of 230xc2x0 C. or lower and a resin blend having a melting point of 230xc2x0 C. or lower. The oxidizable resin is preferably a polyamide resin containing at least 50% by weight of a metaxylylene diamine adipate unit. The resin with the melting point lower than 230xc2x0 C. may not clearly show that point, however, the melting point in this range is included in the scope of the present invention.
The melting point of MXD6 is usually at least 240xc2x0 C. and formed mainly of a crystalline portion. However, an amorphous portion can be increased by copolymerization with another component or by blending with another resin. Crystallization of the amorphous portion in the MXD6 can be prevented to obtain a stable oxygen-absorbing ability.
The melting point of the resin component is decreased by copolymerization with another component or blending with another resin. According to the present invention, it is required that the melting point of the resin component is 230xc2x0 C. or lower, preferably 225xc2x0 C. or lower, and more preferably 220xc2x0 C. or lower.
The present invention is further an oxygen absorber formed of an oxygen-absorbing resin composition including a resin component and a metal catalyst. The resin component has a heat of fusion of 35 J/g or lower.
The heat of fusion is generally used as an indicator of crystallinity of resin. MXD6 usually has a heat of fusion of approximately 50 J/g. A crystalline portion occupies a major part of the MXD6, however, an amorphous portion thereof can be increased by copolymerization with another component or blending with another resin. Moreover, crystallization of the amorphous portion in the MXD6 can be prevented to obtain a stable oxygen-absorbing ability.
The crystalline region of the resin component is decreased by copolymerization with another component or by blending with another resin. The heat of fusion can be measured by DSC described in conjunction of examples below. The heat of fusion is basically proportional to the size of the crystalline region.
In addition to the original crystalline region of the resin component, a crystalline region generated in the temperature-rising process of DSC measurement can be evaluated by means of the heat of fusion. When a resin material is used as an oxygen absorber for a long term, crystallization gradually proceeds. Then, the oxygen-absorbing ability is likely to deteriorate which can also be evaluated simultaneously with the evaluation of the crystalline region. Accordingly, it has been confirmed that the heat of fusion serves as an excellent indicator of the relation between the presence of the crystalline region and the oxygen-absorbing ability. According to the present invention, the heat of fusion of the resin component is required to be 35 J/g or lower, preferably 30 J/g or lower, and more preferably 25 J/g or lower.
Still more preferably, the glass transition temperature (Tg) measured in an atmosphere of 25xc2x0 C. and 65% is 50xc2x0 C. or lower. When Tg is 50xc2x0 C. or lower, crystallization is less likely to occur so that the oxygen-absorbing efficiency can be enhanced.
The resin as discussed above can be produced by copolymerization of a monomer with a metaxylylene adipamide unit with other monomers with diamine unit, dicarboxylic unit or amino carboxylic unit, for example.
Examples of the diamine which can be used for copolymerization are aliphatic diamines such as ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 2,2,4 (or 2,4,4)-trimethylhexamethylenediamine, heptamethylenediamine, octamethylenediamine, nonamethylenediamine, decamethylenediamine, undecamethylenediamine, dodecamethylenediamine, tridecamethylenediamine, hexadecamethylenediamine, and octadecamethylenediamine, alicyclic diamines such as cyclohexanediamine, 1,3-bis aminomethylcyclohexane, bis-(4,4xe2x80x2-aminocyclohexyl)methane, cyclohexanediamine, methylcyclohexanediamine, and bis-(4,4xe2x80x2-aminocyclohexyl)methane, and aromatic diamines such as paraxylylenediamine.
Examples of the dicarboxylic acid component are aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecandioic acid, dodecandioic acid, tridecandioic acid, tetradecandioic acid, hexadecandioic acid, hexadecendioic acid, octadecandioic acid, octadecendioic acid, eicosandioic acid, eicocendioic acid, docosandioic acid and 3,2,4-trimethyl adipic acid, alicyclic dicarboxylic acid such as 1,4-cyclohexane dicarboxylic acid, and aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, xylylene dicarboxylic acid, and naphthalene dicarboxylic acid. Moreover, a polyamide copolymer with lactams such as xcex5-caprolactam and laurolactam, or amino acids such as amino hexanoic acid and amino undecane acid may be used. Among them, copolymerization with nylon 6, 10 component is particularly preferable.
Alternatively, a polyester amido structure can be produced by combination of dicarboxylic acids with diol such as ethylene glycol, butanediol, hexanediol, octanediol, nonanediol, decanediol, diethylene glycol, triethylene glycol, polyethyleneglycol, and tetramethylene glycol.
Any copolymer structure having another structure may be used. Preferably, such structure includes, in order to exhibit an excellent oxygen-absorbing ability as discussed above, as a constitutional repeating unit, at least 50% by weight of a metaxylylene adipamide unit. If the amount of metaxylylene adipamide unit is less than 50% by weight, a satisfactory oxygen-absorbing speed may not be attained. The content of metaxylylene adipamide unit is preferably at least 60 wt % and more preferably 70 wt %.
Instead of the copolymers described above, a blend of an MXD6 polymer and another polymer may be employed. Another polyamide having a good affinity with the MXD6 polymer is suitable for the polymer to be blended.
Specific examples are aliphatic polyamides such as polycaprolactam (nylon 6), polylaurolactam (nylon 12), polyhexamethylene adipamide (nylon 6, 6), polyhexamethylene azelamide (nylon 6, 9), polyhexamethylene sebacamide (nylon 6, 10), and polyhexamethylene dodecanoamide (nylon 6, 12), alicyclic polyamide such as polyamide of 1,3-bis (aminomethyl) cyclohexane and aliphatic dicarboxylic acid, aromatic polyamides such as polyhexamethylene terephthalic amide (nylon 6, T), polyhexamethylene isophthalic amide (nylon 6, I) and polyphenylene phthalic amide, nylon 12-based elastomer, polyether ester amide, polyether polyamide and the like.
The object can be achieved by blending with another resin such as polyester, polyolefin and polyvinyl alcohol to the extent that the oxygen-absorbing property does not remarkably change.
Even with such blending, an excellent oxygen-absorbing ability is exhibited. Therefore, preferably at least 50% by weight of metaxylylene adipamide unit is contained as the constitutional repeating unit. If the content of the metaxylylene adipamide unit is less than 50 wt %, a sufficient oxygen-absorbing speed may not be attained. The content of the metaxylylene adipamide unit is more preferably at least 60 wt %, and still more preferably at least 70 wt %.
The copolymerization and polymer blending as described above allow a resultant resin component to have a melting point of 230xc2x0 C. or lower, a heat of fusion of 35 J/g or lower, or Tg of 50xc2x0 C. or lower. Preferably, when at least 50% by weight of the metaxylylene adipamide unit is contained, a material exhibiting an excellent oxygen-absorbing ability can be obtained.
According to the present invention, the metal catalyst is added to the polymer material or added thereto by being blended therewith. The amount of the added metal catalyst is not particularly restricted. Preferably, the metal catalyst from 0.001 wt % to 10 wt % with respect to the polymer is added. A smaller amount of the added catalyst does not provide improvements of the oxygen-absorbing ability. If a greater amount of the metal catalyst is added, no improvement in the oxygen-absorbing ability is achieved and a problem in terms of molding is likely to occur.
Such an oxygen-absorbing material can be used in the form of powder, particles or film or as one component of a molded product such as container.
In consideration of enhancement of the gas barrier property, a state of high crystallinity is desirable. When there is a strong demand for the gas barrier property of a component of a container such as a hollow container, tray, lid or the like with the purpose of a long-term preservation, preferably an oxygen-absorbing material of high crystallinity with Tg higher than 50xc2x0 C., the melting point higher than 230xc2x0 C., and the heat of fusion higher than 35 J/g is preferably used.
If the oxygen-absorbing ability is required, an oxygen-absorbing material of low crystallinity is preferably used as an oxygen-absorbing material in the form of cloth, nonwoven fabric, powder, particles, film or sheet, or even as a component of a container such as hollow container, tray and lid.
In addition, an oxygen-absorbing material of high crystallinity and an oxygen-absorbing material of low crystallinity are preferably combined for use. Specifically, the oxygen-absorbing material of high crystallinity is used as a component of a container such as hollow container, tray or lid, and the oxygen-absorbing material of low crystallinity is put in the container. Alternatively, a bag, hollow container, tray, lid or the like may be formed as a laminate container having the oxygen-absorbing material of high crystallinity on the outside and the oxygen-absorbing material of low crystallinity on the inside.